An arc furnace that utilizes a combined dc arc system and an ac joule heating system to melt the waste disposed therein and to keep the waste material in a molten condition. The dc arc system utilizes the ac joule heating electrodes as part of the dc current path, thereby eliminating the need for a counter dc electrode. Furthermore, the ac joule heating system provides for peripheral and radial ac current flow to neighboring ac joule heating electrodes, thereby creating a stirring effect in the molten waste. This design greatly simplifies the arc furnace control system while providing a more effective thermal control of the molten waste.
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1. An apparatus for waste conversion said apparatus comprising:
a tank for receiving the waste; a single electrode for supporting a dc arc to melt the waste in the tank, said single electrode being coupled to a source of dc power; a plurality of ac joule heating electrodes, inductively-coupled to a source of ac power, submerged in the melted waste, that maintain the melted waste in a molten state while simultaneously acting as dc counter electrodes, said plurality of ac joule heating electrodes being coupled to said dc power source.
24. A method for converting waste, said method comprising the steps of:
(a) disposing the waste in a tank for treatment; (b) applying a dc arc to melt the waste through a single electrode by coupling said single electrode to a dc power source and adjusting the position of one end of said single electrode with respect to the waste to support said dc arc; (c) applying ac joule heating to maintain the waste in a molten condition through a plurality of ac joule heating electrodes submerged in the waste; and (d) utilizing said plurality of ac joule heating electrodes as dc counter electrodes of said dc arc by: (1) providing a plurality of transformers, each having a primary winding, a pair of secondary windings and a magnetic core, said primary windings being coupled in series to an ac power source and wherein each pair of secondary windings is magnetically coupled to a respective primary winding; (2) winding each pair of secondary windings around said magnetic core with the same number of turns and the same size conductor but winding each one of the pair of secondary windings in a direction for cancelling a dc magnetic flux in said magnetic core when dc current flows in each pair of secondary windings; and (3) connecting a first end of each one of said pair of secondary windings to said dc power source and connecting a second end of each one of said pair of secondary windings to a respective ac joule heating electrode. 2. The apparatus of
a primary winding and a pair of secondary windings wound around a magnetic core; wherein each one of said pair of secondary windings comprises a first end coupled to a respective ac joule heating electrode and wherein each one of said pair of secondary windings comprises a second end coupled to said dc power source; wherein each one of said pair of secondary windings comprises the same number of turns and the same size conductor but each wound in a direction for cancelling a dc magnetic flux in said magnetic core when dc current flows in said pair of secondary windings.
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The present invention generally relates to methods and apparatus for the treatment of waste and more particularly to methods and apparatus for the treatment of waste using arc plasma-joule heated melter systems.
The disposal of municipal solid waste (MSW) and other waste has become a major issue over the past few decades due to space limitations for landfills and problems associated with siting new incinerators. In addition, increased environmental awareness has resulted in a major concern of many large metropolitan areas and to the country as a whole to ensure that the disposal of solid waste is properly handled. See e.g., USA EPA, The Solid Waste Dilemma: An Agenda for Action, EPA/530-SW-89-019, Washington, D.C. (1989).
Attempts have been made to reduce the volume and recover the energy content of MSW through incineration and cogeneration. The standard waste-to-energy incinerator will process the solid combustible fraction of the waste stream, produce steam to drive a steam turbine, and as a result of the combustion process produce a waste ash material. Typically, the ash is buried in a municipal landfill. Current trends and recent rulings, however, may require such material to be shipped to landfills permitted for hazardous waste. This will substantially increase ash disposal costs. Moreover, there is increased public concern about gaseous emissions from landfills and the possibility of contamination of groundwater. Another disadvantage associated with incinerator systems is the production of large quantities of gaseous emissions resulting in the need for costly air pollution control systems in an attempt to decrease emission levels to comply with requirements imposed by regulatory agencies.
In order to overcome the shortcomings associated with incinerator systems, attempts have been made in the prior art to utilize arc plasma torches to destroy toxic wastes. The use of arc plasma torches provides an advantage over traditional incinerator or combustion processes under certain operating conditions because the volume of gaseous products formed from the plasma arc torch may be significantly less than the volume produced during typical incineration or combustion, fewer toxic materials are in the gaseous products, and under some circumstances the waste material can be glassified. It should be understood that the phrases "plasma torch" or "plasma arc torch" are incorrectly used by some as interchangeable with "plasma arc"; plasma torch, or plasma arc torches, are not the same thing as a plasma arc and should not be confused with each other. Moreover, a graphite electrode is used in the present invention, as will be discussed in detail later, to avoid many of the problems and complexities of plasma torches or plasma arc torches.
For example, U.S. Pat. No. 5,280,757 to Carter et al. discloses the use of a plasma arc torch in a reactor vessel to gasify municipal solid waste. A product having a medium quality gas and a slag with a lower toxic element leachability is produced thereby. U.S. Pat. No. 4,644,877 to Barton et al. relates to pyrolytic destruction of polychlorinated biphenyls (PCBs) using a plasma arc torch. Waste materials are atomized and ionized by a plasma arc torch and are then cooled and recombined into gas and particulate matter in a reaction chamber. U.S. Pat. No. 4,431,612 to Bell et al. discusses a hollow graphite electrode transfer arc plasma furnace for treatment of hazardous wastes such as PCBs.
A process for remediation of lead-contaminated soil and waste battery material is disclosed in U.S. Pat. No. 5,284,503 to Bitler et al. A vitrified slag is formed from the soil. Combustible gas and volatized lead, which are formed from the waste battery casings, are preferably transferred to and used as a fuel for a conventional smelting furnace.
The systems proposed by Barton et al, Bell et al, Carter et al, and Bitler et al have significant disadvantages. For example, such disadvantages include insufficient heating, mixing and residence time to ensure high quality, nonleachable glass production for a wide range of waste feeds. Additionally, hearth size and feeder design are significantly limited since furnace walls must be relatively close to the arc plasma which is the only heat source. High thermal stress on the walls of the furnace often occurs as a result of the limitation on the hearth size.
Prior art arc plasma furnaces with metal electrodes further may be limited by short electrode lifetime when used at higher DC current. Therefore, to achieve higher power output, the arc potential must be raised by lengthening the arc. This results in radiative thermal losses to the furnace side walls and leads to metal electrode (torch) ineffectiveness. In addition, there are often difficulties associated with prior art transfer arc plasmas in start-up and restarting of such arc plasma systems when cold, nonelectrically conducting material is being processed.
Another disadvantage associated with prior art systems is the inefficient use of combustible gases produced during conversion of waste materials. For example, combustion of gases often does not result in a high conversion rate and is thus inefficient. In addition, the combustion of such gases frequently results in the emission of pollutants such as oxides of nitrogen (NOx) in amounts that render the process environmentally unattractive.
Thus, while such prior art attempts have been useful, there remains a need in the art for a robust, easy to operate waste conversion system which minimizes hazardous gaseous emissions and which maximizes conversion of a wide range of solid waste into useful energy and produces a product stream which is in a safe, stable form for commercial use or which does not require special hazardous waste considerations for disposal.
It would therefore be desirable to provide robust, user friendly and highly flexible methods and apparatus for processing and converting a wide range of waste materials into useful energy and stable products while minimizing hazardous gaseous emissions, thereby overcoming the shortcomings associated with the prior art.
U.S. Pat. Nos. 6,127,645 (Titus et al.), 6,160,238 (Titus et al.) and 6,215,678 (Titus et al.), all of which are incorporated by reference herein, are directed to overcoming these problems by utilizing AC powered joule heating and DC arc electrode melting in arc plasma-joule heated melter systems. However, there still remains a need to accomplish the operation of the AC powered joule heating and DC arc electrode melting in a more efficient way that uses less components.
An apparatus for waste conversion (e.g., an arc furnace) wherein the apparatus comprises: a tank for receiving the waste; a single electrode for supporting a DC arc to melt the waste in the tank and wherein the single electrode is coupled to a source of DC power; is a plurality of AC joule heating electrodes, coupled to a source of AC power, submerged in the melted waste, that maintains the melted waste in a molten state while simultaneously acting as DC counter electrodes and wherein the plurality of AC joule heating electrodes are coupled to the DC power source.
A method for converting waste wherein the method comprises the steps of: (a) disposing the waste in a tank for treatment; (b) applying a DC arc to melt the waste through a single electrode; (c) applying AC joule heating to maintain the waste in a molten condition through a plurality of AC joule heating electrodes submerged in the waste; and (d) utilizing the plurality of AC joule heating electrodes as DC counter electrodes of the DC arc.
Referring now in detail to the various figures of the drawing wherein like reference characters refer to like parts, there is shown at 20, a scalable DC arc/AC joule heating furnace. The DC arc system melts the waste (e.g., municipal waste, medical waste, etc.) into a molten waste or "glass" (a term used in the waste industry to refer to the melted waste material and not to be confused with the common definition of the word "glass") whereas the AC joule heating system maintains the waste in a molten state. By way of example only, temperatures created by the DC arc system may exist at 6500°C C. whereas the temperatures created by the AC joule heating system may exist at 1000°C C. to 3000°C C.
As will be discussed in detail later, one of the key distinctions between the present invention and conventional arc furnace systems is that the present invention integrates the AC joule heating system with the DC arc system. This totally integrated arc furnace control system provides efficient operation of an arc furnace by allowing the operator to set the operating conditions of both the AC joule heating system and the DC arc system depending on the make-up of the actual waste to be destroyed or portions of the waste that need to be re-used as saleable material for re-use. Thus, based on these factors, the operator can set the proper DC arc voltage, DC arc current, AC joule heating while at the same time reducing the amount of energy that would normally be wasted as when fixed AC/DC system limits are used in other conventional arc furnace systems. In addition, this integrated design greatly reduces the number of electrical components involved in the arc furnace power and control systems.
As shown in
Although not shown, it should be understood that the DC arc electrode 26 and all of the AC joule heating electrodes 30A/30B, 32A/32B and 34A/34B are vertically displaceable through the roof 28, i.e., their position relative to the molten waste 10 can be adjusted. In particular, the DC arc electrode 26 is automatically adjusted through drives (not shown) whereas the AC joule heating electrodes 30A/30B, 32A/32B and 34A/34B are manually adjustable.
As shown in
As mentioned earlier, the arc furnace 20 also comprises an "off gas" treatment tank 24. Since the basic function of the arc furnace 20 is to destroy raw waste, it is desirable to treat the furnace "off-gas" in an adjacent tank 24 designed for this purpose. The off-gas is treating by a pair of gas treatment DC electrodes 38 and 40 and, if needed, AC joule heating electrodes (two of which 41 and 43 are shown in
It should be noted that subsequent references to the DC arc system and the DC arc electrode refers to the DC arc system and DC arc electrode 26 in the first tank 22; similarly, all subsequent references to AC joule heating system and AC joule heating electrodes refers to the AC joule heating system and the AC joule heating electrodes 30A/30B, 32A/32B and 34A/34B in the first tank 22. The following discussion is directed to the DC arc system and the AC joule heating system of the first tank 22.
The AC joule heating system used in the first tank 22 uses a plurality of transformers, each having a primary winding PRI and a pair of secondary windings SEC (see FIG. 12), with each secondary winding being coupled to a respective joule heating electrode submerged in the melted waste 10. Thus, the notation PRI-1 and SEC-1 indicates transformer#1 having a primary winding a pair of secondary windings. Moreover, as discussed earlier, the AC joule heating electrodes form pairs, 30A/30B, 32A/32B, and 34A/34B which correspond to pairs of secondary windings for a plurality of transformers pair (see FIG. 13). As also mentioned earlier, AC joule heating electrodes pairs 30A/30B, 32A/32B, and 34A/34B are physically located opposite each other, with the DC arc electrode 26 interposed between them (see FIGS. 11 and 13).
As mentioned earlier, depending on the size of the arc furnace, the number of joule heating electrodes is determined. In the present invention 20, three transformers (PRI-1/SEC 1, PRI-2/SEC-2 and PRI-3/SEC-3) are utilized, thereby requiring six joule heating electrodes. Hence, it should be understood that the six joule heating electrodes 30A/30B, 32A/32B, and 34A/34B are by way of example only and that any even number of joule heating electrodes can be used (see FIGS. 3-5). Furthermore, it should be understood that all of the joule heating electrodes 30A/30B, 32A/32B, and 34A/34B are evenly and uniformly distributed around the center DC arc electrode 26. In addition, the joule heating electrodes 30A/30B, 32A/32B, and 34A/34B are arranged such that those electrodes connected to a pair of secondary windings from a common transformer are located opposite each other. The physical location of each electrode is also selected to "fill in" the gap between waste charge ports 36. As can be seen most clearly in
The DC arc system (
As can also be seen from this schematic, one end 56 of each secondary winding is connected to the DC arc system, through a DC reactor 58 (the importance of which will be discussed later), while the other end of each secondary winding is connected to a respective joule heating electrode submerged in the molten waste 10. The other side of the DC reactor 58 is coupled to the positive terminal of a DC power supply 60 of the DC arc system. Although not required, the DC arc electrode 26 is connected to the negative terminal of the DC arc power supply 60. The DC arc power supply 60 basically comprises an SCR (silicon controlled rectifier) bank, whose input is coupled to three-phase power from the SEC2 3φ windings and whose rectified DC output is provided at the positive and negative terminals indicated in FIG. 11. The operator can set both the DC arc current (e.g., phase-control firing of the SCRs) and the DC arc voltage via the DC arc power supply 60. With particular regard to setting the DC arc voltage, as is well known in the art, the physical positioning of the free end of the DC arc electrode 26 determines the DC arc voltage. Thus, for example, if the operator were to set the DC arc voltage at some predetermined arc voltage, VoltageDC ARC, (depending on the type and density of the waste, whether there is to re-usable waste created therefrom, etc.), a control system (not shown, but may include one such as described in U.S. Pat. No. 4,320,245 (Gaydon et al.) which is incorporated by reference herein) is used to slowly move the DC arc electrode 26 from its initial position (the free end just making contact with the molten waste 10) upward, away from the surface of the molten waste 10 until the set DC arc voltage is achieved. The DC arc electrode 26 is preferably cylindrical and composed of graphite. The diameter of the DC arc electrode 26 increases as the inside diameter of the tank lining 35 increases but not necessarily in direct proportion to the increase of the diameter of the tank lining 35. As mentioned earlier, the DC arc electrode 26 is mounted in the center of the roof 28 of the tank 22 whose refractory lining 35 is preferably cylindrical and is concentric with the DC arc electrode 26.
One of the unique features of the present invention is the use of the DC reactor 58 which provides DC arc stability. The DC reactor 58 stores the requisite energy to maintain the DC arc voltage should the actual DC arc voltage fall below the set level, VoltageDC ARC.
Phase control retardation of the DC arc current may increase the length of time when the arc voltage is greater than the DC rectifier 58 output voltage. It is during this time interval when the DC reactor 58 delivers its stored energy to the DC arc, thereby maintaining a steady DC arc. In particular, when VoltageDC REC≧VoltageDC ARC, then the DC arc power supply 60 is powering the arc and energy is being stored in the DC reactor 58; conversely, when VoltageDC REC<VoltageDC ARC then the DC reactor 58 is discharging and supplying energy to the DC arc, to maintain the DC arc. When the DC reactor 58 is supplying the arc with power, a return path 62 (
The joule heating electrodes also comprise graphite and are preferably smaller than the DC arc electrode 26. As mentioned earlier, each of the joule heating electrodes are equally spaced circumferentially and are radially located between the DC electrode and the inside of the arc furnace lining.
Furthermore, as shown in
As mentioned earlier, with regard to the AC joule heating electrodes, AC current not only flows between corresponding AC joule heating electrodes, e.g., 30A/30B, underneath the DC arc electrode 26 but also to neighboring AC joule heating electrodes of opposite polarity (see FIG. 13). However, it should be understood that it is within the broadest scope of the present invention to include AC current flow between neighboring AC joule heating electrodes without the need to have AC current flow between corresponding AC joule heating electrodes. For example, using the instantaneous polarities shown in
As can be appreciated from the foregoing, the present invention:
utilize joule heating electrodes that simultaneously act as independent AC joule heating electrodes and DC counter electrodes without electrical interference, i.e., there is no DC and AC interaction;
requires only one DC arc electrode and electrode control device regardless of the size of the arc furnace;
requires only one conventional DC arc power supply;
utilizes joule heating electrodes that simultaneously serve two purposes, as conventional AC joule healing electrodes and as DC arc counter electrodes;
operates regardless of how small or how large the arc furnace may be, and that the invention requires only one DC power supply, one SCR type static switch (needed to control the AC joule heating current to any number of joule heating electrodes) and one AC power supply; the only thing that changes is the amount of power each of these power supplies must deliver;
utilizes the joule heating electrodes as DC arc counter electrodes which also is beneficial in that the magnetic {right arrow over (J)}×{right arrow over (B)} forces (where {right arrow over (J)} is the current density and {right arrow over (B)} is the magnetic field) causes the DC arc to remain under the bottom of the DC arc electrode, thereby reducing the furnace lining damage;
uses a physical configuration of one DC arc electrode in the center of the furnace and a number of AC joule heating/DC arc counter electrodes near the inside of the furnace lining, thereby providing considerably more space for raw waste and wherein the joule heating electrodes aid in shielding the furnace lining from arc radiation.
supports the ideal configuration for raw waste destruction: a "doughnut" of waste between the DC arc electrode and the joule heating electrodes;
provides for joule heating/DC counter electrodes that can be replaced more easily than a DC counter electrode on top of, or as an integral part of the bottom furnace lining, thereby resulting in another cost saving and customer benefit.
Without further elaboration, the foregoing will so fully illustrate my invention that others may, by applying current or future knowledge, readily adopt the same for use under various conditions of service.
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